System for selectively coupling an energy source to a load and method of making same

ABSTRACT

A multi-energy storage device system includes an electric drive coupled to a load, a DC link coupled to the electric drive, and a bi-directional voltage converter having an output channel coupled to the DC link and an input channel. A first energy storage device (ESD) is coupled to the input channel, and a switch is coupled to the DC link and to a second ESD. A system controller causes the switch to couple the second ESD to the DC link for delivering energy stored in the second ESD to the electric drive. The system controller also causes the voltage converter to convert a voltage of the first ESD to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second ESD and causes the switch to decouple the second ESD from the DC link.

BACKGROUND

Embodiments of the invention relate generally to drive systems and, morespecifically, to selectively coupling an energy source to a load tosupply energy thereto in a vehicle or non-vehicle system.

Electric vehicles and hybrid electric vehicles are typically powered byone or more energy storage devices, either alone or in combination withan internal combustion engine. In pure electric vehicles, the one ormore energy storage devices powers the entire drive system, therebyeliminating the need for an internal combustion engine. Hybrid electricvehicles, on the other hand, include energy storage device power tosupplement power supplied by an internal combustion engine, whichgreatly increases the fuel efficiency of the internal combustion engineand of the vehicle. Traditionally, the energy storage devices inelectric or hybrid electric propulsion systems include batteries,ultracapacitors, flywheels, or a combination of these elements in orderto provide sufficient energy to power an electric motor.

When two or more energy sources are used to provide power to drivesystem, the energy sources are typically well-suited to providedifferent types of power. A first energy source, for example, may be ahigh specific energy source that is more efficient or economical atproviding long-term power while a second energy source may be a highspecific-power source more efficient at providing short-term power. Thehigh specific-power source may be used to assist the high specificenergy source in providing power to the system during, for example,acceleration or pulsed load events.

Often, the high specific-power source is directly coupled to the directcurrent (DC) link that supplies a voltage to a load. However, control ofthe DC link voltage is dependent on the directly coupled high powersource and can be lower than a desired response. In addition, thetransient power required from the high energy source to be supplied tothe DC link can be higher than a desired response.

Therefore, it is desirable to provide a system that allows selectivecoupling of an energy source to a load to supply energy thereto.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, a multi-energy storagedevice system includes an electric drive coupled to a load, a DC linkcoupled to the electric drive, and a bi-directional voltage converterhaving an output channel coupled to the DC link and an input channel. Afirst energy storage device is coupled to the input channel of thevoltage converter, a switch is coupled to the DC link, and a secondenergy storage device is coupled to the switch. A system controller isconfigured to cause the switch to couple the second energy storagedevice to the DC link such that energy stored in the second energystorage device is delivered to the electric drive and such that avoltage of the second energy storage device is reduced as the energystored therein is delivered to the electric drive. The system controlleralso causes the bi-directional voltage converter to convert a voltage ofthe first energy storage device to a higher voltage and to deliver thehigher voltage to the DC link, wherein the higher voltage is greaterthan the voltage of the second energy storage device and causes theswitch to decouple the second energy storage device from the DC link.

In accordance with another aspect of the invention, a method ofassembling a propulsion energy system includes coupling an outputchannel of a bi-directional voltage converter assembly to a directcurrent (DC) link and coupling a first energy storage device to a firstinput channel of the bi-directional voltage converter assembly. Thebi-directional voltage converter assembly includes a firstbi-directional voltage converter. The method also includes coupling aswitch to the DC link, coupling second energy storage device to theswitch, and coupling a load to the DC link. The load is configured toreceive energy from one of the first energy storage device and thesecond energy storage device via the DC link. The method furtherincludes coupling a controller to the bi-directional voltage converterassembly and to the switch and configuring the controller to cause theswitch to couple the second energy storage device to the DC link suchthat energy stored in the second energy storage device is delivered tothe load and such that a voltage of the second energy storage device isreduced as the energy stored therein is delivered to the load. Thecontroller is also configured to cause the bi-directional voltageconverter to convert a voltage of the first energy storage device to ahigher voltage and to deliver the higher voltage to the DC link, whereinthe higher voltage is greater than the voltage of the second energystorage device and causes the switch to decouple the second energystorage device from the DC link.

In accordance with another aspect of the invention, a non-transitorycomputer readable storage medium having a computer program storedthereon and representing a set of instructions that when executed by acomputer causes the computer to activate a first switch to transfer avoltage stored in a second energy storage device to a direct current(DC) link coupled to a load such that the voltage stored in the secondenergy storage device is delivered to the load as a voltage of thesecond energy storage device is reduced. The instructions also cause thecomputer to control a bi-directional voltage converter to convert afirst voltage of a first energy storage device to a second voltage andto deliver the second voltage to the DC link, wherein the second voltageis greater than the voltage of the second energy storage device andcauses the first switch to deactivate.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated forcarrying out the invention.

In the drawings:

FIG. 1 schematically illustrates an embodiment of a propulsion systemaccording to an embodiment of the invention.

FIG. 2 schematically illustrates another embodiment of a propulsionsystem according to an embodiment of the invention.

FIG. 3 schematically illustrates another embodiment of a propulsionsystem according to an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to vehicle and non-vehicleapplications. Vehicular applications may include pure-electric orhybrid-electric vehicle applications in, for example, on-road andoff-road vehicles, golf cars, neighborhood electric vehicles, forklifts,and utility trucks as examples. Non-vehicular applications may includenon-vehicular types of loads including pumps, fans, winches, cranes, orother motor driven loads. While described with respect to the vehicularapplications, embodiments of invention are not intended to be limited tosuch.

FIG. 1 illustrates a propulsion system 100 according to an embodiment ofthe invention. Propulsion system 100 may be used in electric or hybridvehicle applications. Vehicle propulsion system 100 includes an energysystem 102 and a system controller 104. Energy system 102 includes afirst energy storage device 106, a second energy storage device 108, anda bi-directional DC-DC voltage converter 110 having an input channel 112coupled to first energy storage device 106 and having an output channel114 coupled to a DC link 116. First energy storage device 106 may beused to provide longer-lasting energy while second energy storage device108 may be used to provide higher-power energy for acceleration, forexample. While first energy storage device 106 is illustrated as abattery, another type of energy storage devices such as anultracapacitor, a fuel cell, a flywheel, or the like is alsocontemplated. While second energy storage device 108 is illustrated asan ultracapacitor, another type of energy storage devices such as abattery, a fuel cell, a flywheel, or the like is also contemplated.

First energy storage device 106 is coupled via DC link 116 to a load118, which, according to an embodiment of the invention, is an electricdrive including a DC-AC inverter 120 and a motor or electromechanicaldevice 122. Motor 122 is preferably an AC motor but is not limited assuch. While not shown, it is to be understood that each of a pluralityof motors 122 may be coupled to a respective wheel or other load or thateach motor 122 may be coupled to a differential for distributingrotational power to the wheels or other load.

Generally, in a motoring mode of operation, voltage converter 110 alsoacts to boost the voltage provided by a low voltage side 124 of energysystem 102 to a high voltage side 126 of energy system 102. That is,voltage from first energy storage device 106 is provided to voltageconverter 110 via a bus 128 coupled to input channel 112 thereof on thelow voltage side 124 of energy system 102. The provided voltage isboosted by voltage converter 110 such that the voltage provided to DClink 116 on the high voltage side 126 of energy system 102 is increasedto an operating level of electric drive 118. Electric drive 118 invertsthe voltage on DC link 116 and provides the inverted voltage toelectromechanical device 122.

In an accelerating mode of operation, energy from high-power secondenergy storage device 108 is generally desired to be used instead of orin addition to the voltage provided by first energy storage device 106via voltage converter 110. Accordingly, propulsion system 100 includes aswitch 130 configured to selectively couple and decouple second energystorage device 108 to/from DC link 116. Coupling second energy storagedevice 108 to DC link 116 allows second energy storage device 108 todischarge its stored energy to take advantage of the higher poweravailable therefrom during acceleration. In one embodiment, secondenergy storage device 108 is an ultracapacitor in which its charge isaffected by its voltage. Second energy storage device 108 preferablyincludes a plurality of series- and parallel-connected capacitor cellsin this embodiment where each capacitor cell has a capacitance greaterthan 100 Farads per cell. Because the charge of second energy storagedevice 108 is affected by its voltage, as the voltage in the secondenergy storage device 108 is supplied to DC link 116, the charge ofsecond energy storage device 108 reduces, thus reducing the voltagethereof. As the voltage of second energy storage device 108 reduces, thecorresponding voltage on DC link 116 also reduces accordingly. Whilesecond energy storage device 108 is coupled to DC link 116 via switch130, voltage converter 110 is not free to establish the voltage on DClink 116 without simultaneously charging second energy storage device108. However, by decoupling second energy storage device 108 from DClink 116, voltage converter 110 is free to establish the voltage on DClink 116 without charging second energy storage device 108, which allowsfaster control of the voltage on DC link 116 and lowers the transientpower supplied from first energy storage device 106.

Accordingly, during the accelerating mode operation, system controller104 is programmed to close switch 130 to couple second energy storagedevice 108 to DC link 116. In one embodiment, system controller 104 isprogrammed to cause the voltage on DC link 116 to substantially matchthe state of charge or voltage of second energy storage device 108 priorto closing switch 130 such as, for example, by boosting the voltage fromfirst energy storage device 106. In the illustrated embodiment, switch130 includes a pair of silicon-controlled rectifiers (SCRs) 132, 134coupled together and arranged in an anti-parallel arrangement. While apair of SCRs is shown, it is contemplated that other switching devicesas known in the art may be used. System controller 104 is thusprogrammed to activate SCR 132 (such as by applying a gate voltagethereto) such that voltage from second energy storage device 108 may besupplied to load 118 via DC link 116. Once activated, SCR 132 tends toremain in an “on” or closed state while current flowing therethroughremains above a holding current thereof.

When the acceleration is finished or when a state of charge or voltageof second energy storage device 108 falls to or below a threshold, forexample, system controller 104 is programmed to cause switch 130 tochange to an “off” or deactivated state to decouple second energystorage device 108 from DC link 116. To turn off SCR 132, the currentflowing therethrough should be lowered to below its holding current. Toaccomplish this, system controller 104 is programmed to cause voltageconverter 110 to boost the voltage of first energy storage device 106 toa voltage higher than the voltage of second energy storage device 108.Increasing the voltage on DC link 116 to a voltage higher than thevoltage of second energy storage device 108 in this manner causes thecurrent flowing through SCR 132 to fall below its holding current. Assuch, SCR 132 is caused to turn off, thus decoupling second energystorage device 108 from DC link 116.

In a decelerating mode of operation in which the speed of rotation ofmotor 122 is to be decreased to zero or to a lower speed from itscurrent speed, system controller 104 is programmed to operate electricdrive 118 in a regenerative mode (such as by operating electromechanicaldevice 122 in a generator mode), wherein electric power or energy isreturned to DC link 116 through DC-AC inverter 120 during a regenerativebraking event. According to embodiments of the invention, systemcontroller 104 causes the regenerative braking energy to be delivered tosecond energy storage device 108 via switch 130 to increase its state ofcharge or voltage. That is, system controller 104 is programmed toactivate SCR 134 (such as by applying a gate voltage thereto) such thatvoltage from load 118 may be supplied to second energy storage device108 via DC link 116. Similar to SCR 132, once activated, SCR 134 tendsto remain in an “on” or closed state while current flowing therethroughremains above a holding current thereof.

When the deceleration is finished or when a state of charge or voltageof second energy storage device 108 rises to or above a threshold, forexample, system controller 104 is programmed to cause switch 130 tochange to an “off” or deactivated state to decouple second energystorage device 108 from DC link 116. To turn off SCR 134, the currentflowing therethrough should be lowered to below its holding current. Toaccomplish this, in one embodiment, propulsion system 100 includes adynamic retarder 136 coupled to DC link 116 that has a low-resistanceresistor 138 and switch 140. System controller 104 is programmed toclose or modulate switch 140 such that the current flowing through SCR134 falls below its holding current. As such, SCR 134 turns off, thusdecoupling second energy storage device 108 from DC link 116. Systemcontroller 104 may then open switch 140 to prevent current fromcontinuing to flow through dynamic retarder 136.

In another embodiment, system controller 104 may cause voltage converter110 to draw current from DC link 116 such that the current flowingthrough SCR 134 falls below its holding current. As such, SCR 134 iscause to turn off, thus decoupling second energy storage device 108 fromDC link 116.

In another embodiment, DC-AC inverter 120 controls reduce the DC-ACinverter's DC output voltage to below that of energy storage device 108,thus causing SCR 134 to turn off and thus decoupling energy storagedevice 108 from DC link 116.

Closed loop power control may be used to determine the power splitbetween the energy sources 106, 108, and (if present) dynamic retarder136.

FIG. 2 illustrates a propulsion system 142 according to anotherembodiment of the invention. Elements and components common to tractionsystems 100 and 142 will be discussed relative to the same referencenumbers as appropriate.

In addition to components 102-140 common with propulsion system 100,propulsion system 142 includes a charge circuit 144 coupled to DC link116 and coupled to second energy storage device 108. If second energystorage device 108 is decoupled from DC link 116, system controller 104is programmed to cause charge circuit 144 to charge second energystorage device 108 from the voltage on DC link 116 such that the fullstate of charge or voltage of second energy storage device 108 may bereached. System controller 104 may optimize the charging of secondenergy storage device 108 such that the effect on first energy storagedevice 106 is minimized. Additionally, charge circuit 144 may be usedduring the regenerative braking event to charge second energy storagedevice 108 using a more controlled operation than merely causing SCR 134to conduct and when there is a desire to maintain a higher DC linkvoltage than the present capacitor voltage. A preferred embodiment ofcharge circuit 144 would be a buck/boost converter allowing controlledcharging of energy storage device 108 from a voltage below that of theDC link to a voltage above the DC link.

FIG. 3 illustrates a propulsion system 146 according to anotherembodiment of the invention. Elements and components common to tractionsystems 100, 142, and 146 will be discussed relative to the samereference numbers as appropriate.

FIG. 3 shows that charge circuit 144 may be coupled directly to firstenergy storage device 106 rather than to DC link 116 as shown in FIG. 2.In this embodiment, the losses of the boost converter may be avoidedduring charging of storage device 108. A preferred embodiment utilizesboost or buck boost chargers to allow the voltage of storage device 108to be charged from a voltage above or below that of storage device 106to a voltage above that of storage device 106.

One skilled in the art will appreciate that system controller 104 may beimplemented via a plurality of components such as one or more ofelectronic components, hardware components, and/or computer softwarecomponents. These components may include one or more tangible computerreadable storage media that generally stores instructions such assoftware, firmware and/or assembly language for performing one or moreportions of one or more implementations or embodiments. Examples of atangible computer readable storage medium include a recordable datastorage medium and/or mass storage device. Such tangible computerreadable storage medium may employ, for example, one or more of amagnetic, electrical, optical, biological, and/or atomic data storagemedium. Further, such media may take the form of, for example, floppydisks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/orelectronic memory. Other forms of tangible computer readable storagemedia not listed may be employed with embodiments of the invention.

A number of such components can be combined or divided in animplementation of the systems described herein. Further, such componentsmay include a set and/or series of computer instructions written in orimplemented with any of a number of programming languages, as will beappreciated by those skilled in the art.

A technical contribution for the disclosed method and apparatus providesfor a computer-implemented device capable of selectively coupling anenergy source to a load to supply energy thereto in a vehicle ornon-vehicle system.

Therefore, according to an embodiment of the invention, a multi-energystorage device system includes an electric drive coupled to a load, a DClink coupled to the electric drive, and a bi-directional voltageconverter having an output channel coupled to the DC link and an inputchannel. A first energy storage device is coupled to the input channelof the voltage converter, a switch is coupled to the DC link, and asecond energy storage device is coupled to the switch. A systemcontroller is configured to cause the switch to couple the second energystorage device to the DC link such that energy stored in the secondenergy storage device is delivered to the electric drive and such that avoltage of the second energy storage device is reduced as the energystored therein is delivered to the electric drive. The system controlleralso causes the bi-directional voltage converter to convert a voltage ofthe first energy storage device to a higher voltage and to deliver thehigher voltage to the DC link, wherein the higher voltage is greaterthan the voltage of the second energy storage device and causes theswitch to decouple the second energy storage device from the DC link.

According to another embodiment of the invention, a method of assemblinga propulsion energy system includes coupling an output channel of abi-directional voltage converter assembly to a direct current (DC) linkand coupling a first energy storage device to a first input channel ofthe bi-directional voltage converter assembly. The bi-directionalvoltage converter assembly includes a first bi-directional voltageconverter. The method also includes coupling a switch to the DC link,coupling second energy storage device to the switch, and coupling a loadto the DC link. The load is configured to receive energy from one of thefirst energy storage device and the second energy storage device via theDC link. The method further includes coupling a controller to thebi-directional voltage converter assembly and to the switch andconfiguring the controller to cause the switch to couple the secondenergy storage device to the DC link such that energy stored in thesecond energy storage device is delivered to the load and such that avoltage of the second energy storage device is reduced as the energystored therein is delivered to the load. The controller is alsoconfigured to cause the bi-directional voltage converter to convert avoltage of the first energy storage device to a higher voltage and todeliver the higher voltage to the DC link, wherein the higher voltage isgreater than the voltage of the second energy storage device and causesthe switch to decouple the second energy storage device from the DClink.

According to yet another embodiment of the invention, a non-transitorycomputer readable storage medium having a computer program storedthereon and representing a set of instructions that when executed by acomputer causes the computer to activate a first switch to transfer avoltage stored in a second energy storage device to a direct current(DC) link coupled to a load such that the voltage stored in the secondenergy storage device is delivered to the load as a voltage of thesecond energy storage device is reduced. The instructions also cause thecomputer to control a bi-directional voltage converter to convert afirst voltage of a first energy storage device to a second voltage andto deliver the second voltage to the DC link, wherein the second voltageis greater than the voltage of the second energy storage device andcauses the first switch to deactivate.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. A multi-energy storage device system comprising:an electric drive coupled to a load; a direct current (DC) link coupledto the electric drive; a bi-directional voltage converter comprising anoutput channel coupled to the DC link and comprising an input channel; afirst energy storage device coupled to the input channel of thebi-directional voltage converter; a switch coupled to the DC link; asecond energy storage device coupled to the switch; and a systemcontroller configured to: cause the switch to couple the second energystorage device to the DC link such that energy stored in the secondenergy storage device is delivered to the electric drive and such that avoltage of the second energy storage device is reduced as the energystored therein is delivered to the electric drive; and cause thebi-directional voltage converter to convert a voltage of the firstenergy storage device to a higher voltage and to deliver the highervoltage to the DC link, wherein the higher voltage is greater than thevoltage of the second energy storage device and causes the switch todecouple the second energy storage device from the DC link.
 2. Thesystem of claim 1 wherein the system controller is further configuredto: cause the switch to couple the second energy storage device to theDC link such that energy generated in the electric drive is delivered tothe second energy storage device and such that a voltage of the secondenergy storage device is increased as the energy is delivered theretofrom the electric drive; and cause the bi-directional voltage converterto lower a current flowing through the switch to cause the switch todecouple the second energy storage device from the DC link.
 3. Thesystem of claim 1 wherein the system controller is further configuredto: cause the switch to couple the second energy storage device to theDC link such that energy generated in the electric drive is delivered tothe second energy storage device and such that a voltage of the secondenergy storage device is increased as the energy is delivered theretofrom the electric drive; and cause the inverter to lower a voltage onthe DC link to a voltage lower than a voltage of the second energystorage device to cause the switch to decouple the second energy storagedevice from the DC link.
 4. The system of claim 1 further comprising adynamic retarder coupled to the DC link; and wherein the systemcontroller is further configured to: cause the switch to couple thesecond energy storage device to the DC link such that energy generatedin the electric drive is delivered to the second energy storage deviceand such that a voltage of the second energy storage device is increasedas the energy is delivered thereto from the electric drive; and causethe dynamic retarder to lower a current flowing through the switch tocause the switch to decouple the second energy storage device from theDC link.
 5. The system of claim 4 wherein the dynamic retardercomprises: a resistor; and a switching device.
 6. The system of claim 1wherein the switch comprises a pair of silicon controlled rectifierscoupled together and arranged in an anti-parallel configuration.
 7. Thesystem of claim 1 further comprising a charge circuit coupled to thesecond energy storage device; and wherein the system controller isfurther configured to cause the charge circuit to increase the voltageof the second energy storage device if the second energy storage deviceis decoupled from the DC link.
 8. The system of claim 7 wherein thecharge circuit is further coupled to the DC link and configured toconvert a voltage of the DC link to a charging voltage and to supply thecharging voltage to the second energy storage device to increase thevoltage thereof.
 9. The system of claim 7 wherein the charge circuit isfurther coupled to the first energy storage system and configured toconvert a voltage of the first energy storage system to a chargingvoltage and to supply the charging voltage to the second energy storagedevice to increase the voltage thereof.
 10. The system of claim 1wherein the system controller is further configured to cause thebi-directional voltage converter to boost a voltage from the firstenergy storage device and to supply the boosted voltage to the DC linkprior to causing the switch to couple the second energy storage deviceto the DC link, wherein the boosted voltage is substantially equal to avoltage of the second energy storage device.
 11. The system of claim 1wherein the first energy storage device comprises a battery; and whereinthe second energy storage device comprises an ultracapacitor comprisinga plurality of series- and parallel-connected capacitor cells, eachcapacitor cell having a capacitance greater than 100 Farads.
 12. Amethod of assembling a propulsion energy system, the method comprising:coupling an output channel of a bi-directional voltage converterassembly to a direct current (DC) link, the bi-directional voltageconverter assembly comprising a first bi-directional voltage converter;coupling a first energy storage device to a first input channel of thebi-directional voltage converter assembly; coupling a switch to the DClink; coupling second energy storage device to the switch; coupling aload to the DC link, the load configured to receive energy from one ofthe first energy storage device and the second energy storage device viathe DC link; coupling a controller to the bi-directional voltageconverter assembly and to the switch; and configuring the controller to:cause the switch to couple the second energy storage device to the DClink such that energy stored in the second energy storage device isdelivered to the load and such that a voltage of the second energystorage device is reduced as the energy stored therein is delivered tothe load; and cause the bi-directional voltage converter to convert avoltage of the first energy storage device to a higher voltage and todeliver the higher voltage to the DC link, wherein the higher voltage isgreater than the voltage of the second energy storage device and causesthe switch to decouple the second energy storage device from the DClink.
 13. The method of claim 12 further comprising configuring thecontroller to set a voltage of the DC link via boost control of thevoltage of the first energy storage device.
 14. The method of claim 12wherein the first energy storage device comprises a battery; and whereinthe second energy storage device comprises an ultracapacitor.
 15. Themethod of claim 14 wherein the ultracapacitor is configured to provide ahigher power than the battery.
 16. The method of claim 12 furthercomprising: coupling a dynamic retarder to the DC link; and configuringthe controller to: cause the load to operate in a regenerative brakingmode and to supply a regenerative braking voltage to the DC link; causethe switch to couple the second energy storage device to the DC linksuch that the regenerative braking voltage is delivered to the secondenergy storage device and such that a voltage of the second energystorage device is increased as the regenerative braking voltage isdelivered thereto; and cause the dynamic retarder to lower a currentflowing through the switch to cause the switch to decouple the secondenergy storage device from the DC link.
 17. The method of claim 12wherein coupling the switch to the DC link comprises coupling a pair ofsilicon controlled rectifiers arranged in an anti-parallel configurationto the DC link.
 18. The method of claim 12 further comprising coupling acharge circuit the second energy storage device; and configuring thecontroller to cause the charge circuit to increase the voltage of thesecond energy storage device if the second energy storage device isdecoupled from the DC link.
 19. A non-transitory computer readablestorage medium having a computer program stored thereon and representinga set of instructions that when executed by a computer causes thecomputer to: activate a first switch to transfer a voltage stored in asecond energy storage device to a direct current (DC) link coupled to aload such that the voltage stored in the second energy storage device isdelivered to the load as a voltage of the second energy storage deviceis reduced; and control a bi-directional voltage converter to convert afirst voltage of a first energy storage device to a second voltage andto deliver the second voltage to the DC link, wherein the second voltageis greater than the voltage of the second energy storage device andcauses the first switch to deactivate.
 20. The computer readable storagemedium of claim 19 wherein the set of instructions further causes thecomputer to: activate the first switch to transfer a voltage on the DClink generated by the load to the second energy storage device such thatthe voltage of the second energy storage device is increased; andactivate a dynamic retarder switch in a dynamic retarder coupled to theDC link to reduce a current flowing through the switch to a level lessthan a holding current of the first to deactivate the first switch. 21.The computer readable storage medium of claim 20 wherein the set ofinstructions further causes the computer to activate a charge circuitcoupled to the second energy storage device when the second energystorage device is decoupled from the DC link to cause the charge circuitto increase the voltage of the second energy storage device.